Surge-modifying lightning arrester construction

ABSTRACT

A valve-type lightning arrester construction includes a fastresponsive, magnetically saturable inductive member effective to facilitate uniform distribution of surge current discharging through the bound silicon-carbide blocks of the current-limiting valve material forming a part of the arrester, the improved current distribution throughout the blocks increasing the magnitude of the current which can be discharged through the blocks and concurrently decreasing block damage attending &#39;&#39;&#39;&#39;fulgurite channeling.&#39;&#39;&#39;&#39; An additional desideratum is a greater degree of protection from damage by superimposed surges, such as are initiated by lightning strokes to an exposed overhead electrical conductor, to connected cables or apparatus at points of electrical discontinuity, e.g., at the junction of an overhead line conductor with an underground cable.

United States Patent [72] Inventor Ralph R. Pittman 1015 LouisianaStreet, Little Rock, Ark. 72202 [21] Appl. No. 44,681

[22] Filed June 9, 1970 [45] Patented Dec. 28, 1971 [54] SURGE-MODIFYINGLIGHTNING ARRESTER [56] References Cited UNITED STATES PATENTS 2,158,8595/1939 Horikoshi 317/66 2,973,490 2/1961 Schlicke 336/175 X 3,072,8151/1963 Machler 317/61 X Primary Examiner-Gerald Goldberg AssistantExaminer-Harvey Fendclman Att0meyRa1ph R. Pittman ABSTRACT: A valve-typelightning arrester construction includes a fast-responsive, magneticallysaturable inductive member effective to facilitate uniform distributionof surge current discharging through the bound silicon-carbide blocks ofthe current-limiting valve material forming a part of the arrester, theimproved current distribution throughout the blocks increasing themagnitude of the current which can be discharged through the blocks andconcurrently decreasing block damage attending "fulgurite channeling."An additional desideratum is a greater degree of protection from damageby superimposed surges, such as are initiated by lightning strokes to anexposed overhead electrical conductor, to connected ca-- bias orapparatus at points of electrical discontinuity, e.g., at the junctionof an overhead line conductor with an underground cable.

25 /|2 EXPOSED OVERHEAD CONDUCTOR l3 UNDERGROUND CABLE TO GROUNDED Itsg? CABLE SHEATH PATENTED M028 I97! SHEET 3 OF 4 2a 25 I2 EXPOSEDOVERHEAD CONDUCTOR PIC-5.6

TO GROUNDED INVENTOR; 1? 5w. 7"" "1 BY CABLE SHEATH FIG 5 PATENIEDnzczsmn" O SHEET & [1F 4 25 l2 EXPOSED OVERHEAD CONDUCTOR m ,a m m c w 4W W. w w 0 VJ m R 7? M m m HG B\ H Em DE NH ws E w M i [a w .m. mu inI|\ 9 4 d 7 If: a! W.\ H G 6 l 1/11 l 3 7 w F 4/ mm SURGE-MODIFYINGLIGHTNING ARRESTER CONSTRUCTION BACKGROUND This invention relatesgenerally to lightning arresters, and specifically to the constructionof a valve-type arrester having improved uniformity of surge currentdistribution in the valve material, along with protectivecharacteristics which are not adversely affected by the electricalproperties of the connected circuits when applied at the points ofelectrical discontinuity of a circuit.

A lightning arrester is employed in electrical power systems to providea low-impedance path to ground and thereby preclude the occurrence ofthe application of excessive and damaging voltages to the insulation ofconnected apparatus, such as might be caused by lightning or switchingsurges, and, at the same time, to present an almost open circuit path toground under normal conditions. The ideal lightning arrester would havethe ability to instantaneously switch from an open circuit condition toa zero-resistance condition to dissipate abnormal surges, and instantlyswitch back to the open-circuit condition when normal operatingconditions again prevail.

Currently available valve-type arresters are a compromise betweensparkover voltage and ampacity with accompanying voltage drop, and themaximum voltage stress across the protected insulation is either thesparkover voltage or the voltage drop accompanying the surge currentpassage through the current-limiting resistors which make up the valvestructure.

The nonlinear resistance or valve material is usually silicon carbide,made in the form of blocks with some binding material. The mostimportant deficiency of the material is its inability to reduce inresistance with a precipitousness compatible with the rate of rise withtime of the applied surge current, which, in the case of lightningsurges, may rise at rates of the order of 5,000 amperes per microsecond,and may exceed l00,000 amperes in magnitude. Because of the small butimportant inherent nonuniformity of the silicon carbide valve material,a suddenly applied current may pass through only a particular region ofthe valve block, thermally fusing the material to a permanently lowresistance path. Arresters are thus destroyed by such hot channels" andthe resulting fulgurites evidence such destruction.

Valve arresters are intended to function; i.e., discharge, at somepredetermined voltage within some predetermined time of application ofthe voltage, this voltage level" often being referred to as theprotective level," and the protected insulation, if it has an electricalstrength exceeding the protective level" will be protected.

This simple analysis neglects the connected-circuit electricalproperties, which substantially affect the stress not only on theconnected arrester but also on the protected insulation. As an example,an overhead line terminated at a transformer is practically a 100percent positive surge reflection situation, the surge voltagetransmitted by the line almost doubling in magnitude unless quicklylimited by arrester discharge. As another example, an overhead lineterminal at an underground cable is a negative reflection situation, thecharging of the capacitance of the cable reducing the voltage at thejunction while the charging takes place, thus tending to preventarrester operation and permitting a voltage surge to enter the cable,wherein it may double at some open-circuit point and thereby overstressthe cable insulation.

The most potentially damaging condition occurs when the magnitude of thecable capacitance is sufficient to hold the surge voltage at a leveljust low enough to prevent arrester sparkover, so that nosurge-to-ground dissipation occurs, the maximum surge energy enteringthe cable to be doubled at some reflection point therealong.

An additionally hazardous situation develops when a transient surge,such as may be caused by a switching operation, travels along anunderground cable until it meets the relatively massive surge impedancebarrier of an overhead line at its point of connection therewith. Hereagain the reflected surge potential adds directly to the original surgevoltage, and this voltage amplification may overstress the insulation ator near the cable terminal.

SUMMARY OF THE INVENTION The arrester construction has as a component aninductor, which also functions as an inductive surge-wave modifier, andwhich is interposed in series circuit relationship with a protectedconductor; i.e., .a conductor which requires some predeterminedexcess-voltage limitation to preclude voltagestress damage to theinsulation of connected lines or apparatus. An important considerationin such a concept is the requirement that the inductor possess theability to instantly respond to superimposed fast-rise surge currentscaused by atmospheric discharges, or otherwise, while beinginsignificantly affected by the normal slow-rise current, such as thatof 60 Hz. frequency.

The extent to which the electrical composite of a superimposed fast-risesurge and the accompanying slow-rise dynamic current is affectedvoltagewise upon meeting a lumped inductance is measured by therespective rates of change of current; i.e., L di/dt, L representing theinductance and di/dt the rate of change of current. Lightning currentsand transient surge currents cause very substantial induced voltages ina very small inductance, because they are characterized by exceedinglyrapid change in amplitude, with rates of rise of the order of thousandsof amperes per microsecond, while the normal dynamic current, with itsrelatively insignificant rate of change, has a corresponding relativelynegligible effect.

As is commonly known, a surge current influenced by an inductive memberis retarded in time to lag behind the impelling voltage, and the rate ofrise delayed while the magnetic core is being saturated, to therebyrequire a slightly longer time to current crest. If a surge current isso altered in character before being applied to an associated valve-typedischarge structure, the impact of the surge on the nonlinearcurrent-limiting resistance blocks will be concurrently reduced by theshort but important increase in time to permit the arrester blocks tobecome more fully conductive.

Absolute uniformity of resistance throughout the volume of a stackedgroup of silicon carbide blocks, while the desired property, is notattainable under practical manufacturing procedures. ln addition,nonuniformity of electrical contact resistance between the blocks, aswell as with the terminal conductors, is the prevailing condition.

Whether in granular form or molded blocks, silicon carbide has aresistance characteristic of decreasing its resistance nonlinearly withimpressed voltage, the rate of increase in conductivity being greatestduring the first microsecond of voltage application. It is during thisinterval that the nonuniformity of precipitously steep current rise mayinitiate thermal channeling through the blocks by melting and therebydestroying the material as a nonlinear current-limiting resistor. Anextremely short-time delay in current rate-of-rise not only alleviateschanneling but also concurrently reduces the voltage drop across theresistors, and this is the voltage normally applied to the insulation tobe protected.

The arrester construction of the invention, when applied with theinductive surge-wave modifier interposed as the connecting memberbetween a terminal of an exposed overhead conductor and a terminal of anassociated underground cable, provides enhanced protection for theinsulation of the cable. Absent the inductive surge-wave modifier, thevoltage impelling a surge current moving from the high surge impedanceoverhead conductor to the relatively low surge impedance of the cablecannot rise to sparkover the gaps of the arrester and dissipate thesurge energy to ground until the capacitance of the cable is charged.The interposition of the inductance of the surge-wave modifier providesa reflecting barrier to the entrance of surge energy into the cable forthe few microseconds prior to magnetic saturation of the core, thisshort-time nullification of the capacitance effect of the cable enablinga more prompt sparkover of the line-to-ground discharge path. Similarly,a surge traveling from the cable to the connected overhead terminal willmeet the lumped inductance of the surge-wave modifier, to facilitateline-toground sparkover.

For optimum operation, the arrester construction herein requires aninductive device capable of magnetically responding practicallyinstantly to current variations therethrough, and particularly when suchvariations are notable for rates of current rise up to 5,000 amperes permicrosecond, such rates being characteristic of transient surgecurrents. To meet this requirement, the magnetic core elements arepreferably ferrites having a high quality or Q-factor at frequencies inthe 200-300 megacycle range. One such material is a sintered nickel-zincferrite containing relatively small amounts of the oxides of cobalt andmanganese, which is described in further detail in U.S. Pat. No.3,036,009.

THE DRAWINGS FIG. 1 is an elevational view of a preferred embodiment ofthe invention, shown partly in section;

FIG. 2 is a diagram of the circuitry appropriate to the embodiment ofFIG. 1;

FIG. 3 is an elevational view of the inductive surge-wave modifier ofthe invention, shown principally in section;

FIG. 4 is a section taken along the line 4-4 of FIG. 3;

FIG. 5 is a second embodiment of the invention, shown partly in section;

FIG. 6 is a circuit diagram appropriate to the embodiment of FIG. 5;

FIG. 7 is a third embodiment of the Invention, also shown partly insection; and

embodiment of FIG. 7 to associated circuitry.

DESCRIPTION OF THE PREFERRED EMBODIMENTS The arrester constructionillustrated at FIG. 1 includes generally a pair of similarvoltage-limiting, arc-quenching discharge members 11, and an inductivesurge-wave modifier 10. In more detail, the arrester of FIG. 1 comprisesa pair of elongate, transversely spaced housings 17 of weather-resistantinsulating material, such as porcelain. Within each housing is disposedthe group of elements which constitute a well-known voltage-responsive,arc-extinguishing structure, including the quench gaps l9 and theserially related current-limiting silicon carbide resistor blocks 18.The ground terminal members 16 extend outwardly from their connectionwith the end resistor block, and in connection with the groundedmounting bracket 14, and provide a ground connection for the dischargepath as indicated symbolically by the conductor 49 and the ground 15.

The line terminals 20 of the discharge structure project outwardly fromthe housings 17 at the respective line ends thereof, an exposed overheadline conductor l2 being connected to one terminal 20 by means of acooperating terminal nut 24, and an underground cable 13 is similarlyconnected to the other terminal 20.

A first L-shaped inductor mounting bracket 45 is rigidly clamped betweenthe top of one insulator by the terminal nut 24, extending firsttransversely toward the other insulator and thence upwardly above theassociated insulator. Similarly, a second L-shaped inductor mountingbracket 44 is rigidly attached to the other insulator, extendingtransversely toward said one insulator and thence upwardly above theinsulator to which it is fastened. The upstanding portions of thebrackets 44 and 45 are disposed in opposed spaced relationship to oneanother, and the inductor l0 bridges the space therebetween,

being rigidly secured at the underground cable terminal end the sealingnut 36 ofthe inductor housing and the terminal nut 25 of the inductor.In similar fashion, the other end of the inductor I0 is rigidly securedat the overhead conductor terminal end by the extension of the threadedstud terminal 43 through the bracket 45 and coaxially into the threadedhoused inductor terminal 39, the bracket 45 being engaged at itsrespective sides by the other sealing nut 36 and the terminal nut 25. Amore detailed description of the inductor 10 will follow in thereference to FIG. 3.

As may be seen by the circuit diagram of FIG. 2, a surge moving from theoverhead conductor I2 to the conductor 3i ofthe underground cable 13, ora surge moving from the cable 13 to the overhead conductor 12, isintercepted by the serially connected inductor l0. Steep-front surgecurrents, upon arrival at a terminal of the inductor l0, will build upan L di/d! voltage to cause some dissipation of the surge to groundthrough the associated discharge member. While this is occurring, theremaining portion of the surge passing through the inductor is modifiedthereby by a retardation of the rate of rise of current, causing arelatively lower voltage sparkover of the gaps of the other dischargemember. In this way the discharge duty of both discharge members isdiminished as the current density in the current-limiting valve materialof both discharge members attains a corresponding current-shared value.As indicated, the ground terminals of both discharge members arepreferably connected to a grounded conducting shield 32 of the cable I3.

With the circuitry of FIG. 2, absent the inductive surgewave modifier10, one or the other, but not both of the surge discharge members wouldsparkover on the front of a transient surge, the remaining dischargemember operating only if and when the voltage drop through thedischarging unit exceeded the sparkover voltage level of thenonfunctioning unit. In addition, neither of the two discharge unitscould operate at all during the period that the cable capacitance wasbeing charged, with the resulting admission into the cable of asubstantial amount of perhaps damaging surge energy.

The inductive surge-wave modifier 10 of the invention is shown more indetail by the section elevation of FIG. 3. An elongate cup-shapedhousing of insulating material 37 is preferably formed of someelastomeric plastic, so that it may be stretched over contained elementsto provide a weathertight assembly. A through conductor 35, which ispreferably copper, extends coaxially within the housing 37, having acoextensive threaded terminal portion projecting therefrom, on which arefitted the metal sealing washer 40, the sealing nut 36 and the terminalnut 25 Within the housing 37, the through conductor 35 extends through acore consisting of a stacked group of rings of magnetic material 38,terminating in threaded engagement with the housed conducting terminal39. A washer 41, of elastomeric material, resiliently engages the outerface of the stacked group, asan additional hermetic seal. The housedterminal 39 is provided with a coaxially extending, internally threadedrecess 42, and as shown in FIG. 4, is also provided with a plurality ofperipherally spaced, outwardly extending protuberances 50, for engagingan end portion of the wall of the housing 37, so that the wall andterminal may be rotated as a unit.

The embodiment shown at FIG, 5 of the drawings includes the samesurge-discharge structure 11 and the same surgewave modifier describedin connection with FIG. I, in a modified configuration. The surge-wavemodifier is here rigidly fixed in coextensive relationship at the lineend of the elongate insulating housing 17, being joined thereto by thethreaded engagement of the housed inductor terminal 39 with theprojecting ter'ininal 20 of the surge discharge member.

One leg of an L-shaped conducting electrode 23 is mechanically heldbetween the line end of the housing 17 and the housed inductor terminal39, being electrically connected thereto and having an end portionextending outwardly to provide for the attachment ofa first lineterminal 24.

The other leg of the Lshaped electrode extends in parallelspacedrelation with the inductor to a point near the outer end thereof. Aconducting line terminal electrode 28 extends transversely toward and inspaced relation with the outer end of the electrode 23 to form theinductor-shunting spark gap 29, being rigidly attached to the outer endof the inductor housing 37 by the threaded engagement of the sealing nut36 with the projecting end of the through-conductor rod 35.

The inductive surge-wave modifier 10 is disposed in series with the lineconductors l2. and 13 by their connection to the line terminals 24 and25, respectively.

The components of the arrester of FIG. 5 are diagramed in FIG. 6, alongwith a symbolic indication of the voltage-influencing capacitances of anoverhead line conductor and an underground line conductor. Asuperimposed surge attempting to move from the exposed conductor 12 intothe cable l3 first meets and begins its travel through and the magneticsaturation of the inductive surge-wave modifier l0. Depending upon thedi/dt of the surge, some or all of it passes through the inductor, andwhatever does so is rendered less hazardous by the concurrent showing ofthe rate of rise. Assuming all of the surge passes through the inductorl0; i.e., that the induced voltage thereacross is inadequate tosparkover the gap 29, the retarded-rise wave will react with the cablecapacitance, shown symbolically at the numeral 34, which per unit oflength is many times greater than that of the overhead line indicated at33. At the same time, the retarded-rise wave is impressed on thedischarge structure 11, causing it to sparkover at a voltage compatiblewith its slowed rate of rise. In the alternate situation in which the Ldi/dt voltage is such that sparkover of the gap 29 occurs, the diyidedsurge is vectorially reunited at the cable end of the inductor, and thecomposite wave similarly treated.

The insulation to be protected may be readily coordinated with thevoltage-limiting qualities of the arrester construction. As an example,an 8 kv., 60 Hz. line to ground cable may have a basic insulation levelof 95 kv. with a surge rising to crest in 1% microseconds and falling toone-half crest value in 40 ms. For protecting such a cable, thesurge-discharge structure may have a sparkover voltage of about 47 kv.,the inductive surgewave modifier may have a magnetic core of six 112-inch diameter, 34-inch thickness ferrite rings, and the inductorshunting gap may have a 36-inch electrode spacing. The dynamic voltagedrop through the inductor at l,000 amperes, 60 Hz. would be about 30volts; the surge-voltage sparkover of the shunting gap about 30 kv.; andthe L di/dt across the surgewave modifier about 39 kv. at a di/dt of 500amperes per microsecond. I

The embodiment illustrated at FIG. 7 also includes the samesurge-discharge member 11 and surge-wave modifier as hereinbeforedescribed, with some additional members added to provide a modifiedcircuitry. In this embodiment, the surgedischarge structure 11 hascoaxially mounted thereon, at the line end, the insulator 21, and thesurge-wave modifier 10 extends outwardly from the outer end of theinsulator. Attachment of the insulator 21 is effected by the threadedengagement of the projecting tenninal of the discharge structure with aninternally threaded, coaxially extending socket insert 22, which isrigidly secured to the insulator, as by cementing. At the other andouter end of the insulator 21, the inwardly secured stud bolt 51projects coaxially therefrom, the projection threadedly engaging thehoused terminal of the inductor 10.

An exposed overhead conductor 12 is indicated, connected to theprojecting end portion of the inductor through conductor 35, and anunderground cable 13 is shown connected to the other terminal of theinductor through the terminal 25. The latter terminal is engaged with atransverse extension of the conducting terminal plate 26, which isrigidly clamped between the respective adjoining ends of the insulator21 and the inductor 10.

An intermediate L-shaped conducting terminal electrode has one legthereof fixed between the adjacent ends of the insulator 21 and thehousing 17, extending first transversely therefrom and thence in spacedrelation along the insulator to a point near its outer end. A portion ofthe terminal plate 26 is extended toward and in spaced relationship withthe outer end of the electrode 48, to provide a spark gap 27, disposedelectrically between the discharge member ll, and that terminal of theinductor which is connected to the underground cable.

A line-terminal L-shaped conducting electrode 46, one leg of which issupported by clamping between the outer end of the inductor housing 37and the sealing nut 36, extends first transversely therefrom and thencein spaced relation with the housing 37 to a point near and spaced fromthe outer end of the intermediate electrode 48, thereby providing thespark gap 30 between the surge discharge member 11, and that terminal ofthe inductor which is connected to the overhead conductor.

The outer end of the intermediate electrode 48 is preferably positionedcoplanar with the terminal plate electrode 26, so that the preferentialspark paths will be either across the gap 27 or the gap 30, which arepreferably of equal length, the longer gap 47 being of such a length asto constitute an unpreferential spark relative to the others.

FIG. 8 is a circuit diagram of the arrester of FIG. 7, along with somecircuit connections. This diagram shows that a surge wave arriving ateither end of the surge-wave modifier; i.e., either from the overheadline or the cable, will be accorded identical treatment with respect toits dissipation to ground through either, or both, of the spark gaps 27and 30, and the discharge member 11. While the overhead line is shownconnected to the outer line terminal and the cable to the other lineterminal, it may be seen that they may be interchanged without affectingthe operation of the arrester of this embodiment.

Common to the embodiments of the lightning arrester construction hereindisclosed is a three-terminal surge-voltage protective device includingtwo line terminals and a ground terminal, and a valve-type,voltage-limiting arc-extinguishing discharge member providing apreferred sparkover path to the ground terminal from either or both ofthe line terminals, there being a magnetically saturable, fast-responsereactor interposed between the line terminals and in series circuitrelation with the conductors, the surge voltage of which is to belimited. Also common to the arrester constructions herein presented is astructure embodying means for magnetically modifying transientelectrical surge waves to inhibit their capability to destroy thesilicon carbide components of a valve-type lightning arrester, thisprocedure functioning concurrently to enhance its protective qualities.

What is claimed is:

l. A valve-type lightning arrester construction comprising an elongatehousing of insulating material containing a surgedischarge structurewhich includes a quench-spark-gap portion and at least one block ofnonlinear resistance material disposed in series circuit relationbetween a line terminal at one end of said housing and a ground terminalat the other end of said housing, and surge-modifying means mountedexternally on the line terminal end of said housing and coextensivetherewith, said surge-modifying means including a casing of insulatingmaterial having an elongate cavity therein. a rectilinearthrough-conductor extending longitudinally through said casing, terminalmeans at the respective ends of said casing, a nickel-zinc ferrite coredisposed within the cavity and surrounding the through-conductor, andmeans connecting said surge-modifying means in series circuit relationwith said surge-discharge structure, said surge-modifying means beingeffective to inductively alter the current-time relationship ofelectrical surges discharging from the line terminal to the groundterminal before contacting any part of said discharge structure.

2. A method for reducing fulgurite channeling through the nonlinearresistance blocks in the discharge path of a valvetype lightningarrester while the arrester is connected to an electrical conductorwhich transmits to the arrester an electrical composite of normaldynamic energy and a surge wave of abnormal superimposed energy,comprising the step of retardingly modifying the current-rate-of-rise ofthe surge wave at its point of entrance to the discharge path by passingthe surge wave through a single-tum, magnetically saturableferrite-cored surge-wave modifier.

3. A valve-type lightning arrester construction comprising a pair ofelongate, transversely spaced housings of insulating material containingelements constituting a voltage-responsive arc-extinguishing dischargestructure including current-limiting silicon-carbide resistors each ofwhich is susceptible to thermal damage by the sudden passage of acurrent surge therethrough within a predetermined time interval suchthat time for current deconcentration and heat dispersion from theinitial current-carrying channel is unavailable, a first line terminalat one end of one of said housings connected to the discharge structuretherein, a second line terminal at the adjacent one end of the other ofsaid housings connected to the discharge structure therein, commonground-terminal means at the respective other ends of said housingsconnected to the respective discharge structures therein, and aninductive surge-wave modifier mechanically joining and electricallyconnecting said first line terminal to said second line terminal.

4. The arrester construction defined in claim 3, in which the inductivesurge-wave modifier includes a magnetic core comprising a stacked groupof discrete rings of magnetic material.

5. The arrester construction as in claim 3, wherein the inductivesurge-wave modifier includes a magnetic core comprising a stacked groupof discrete rings of ceramic ferrite material.

6. The arrester construction as in claim 3, in which the inductivesurge-wave modifier includes a core comprising a stacked group ofdiscrete rings of sintered nickel-zinc ferrite containing a relativelysmall proportion of the oxides of cobalt and manganese.

7. A valve-type surge arrester comprising a pair of adjacent elongateinsulator housings each containing a similar series circuit assembly ofcurrent-limiting resistors and spark gaps, spaced line terminals at onepair of adjacent ends of said housings and spaced ground terminals atthe other pair of adjacent ends of said housings, conducting meanselectrically connecting said spaced ground terminals, and conductingmeans electrically connecting said spaced line terminals, saidlast-named conducting means including a saturable inductive surge-wavemodifier effective to implement surge sparkover of one of said seriescircuit assemblies and concurrently facilitate initiation of sparkoverof the other of said series circuit assemblies.

8. A valve-type lightning arrester construction comprising an elongatehousing of insulating material, a first conducting line terminaldisposed at one end of said housing and a conducting ground terminaldisposed at the other end of said housing, and elements constituting apreferential electrical discharge path interposed between said line andsaid ground terminals, said elements including a plurality of boundsilicon carbide blocks subject to thermal damage upon the passagetherethrough of a current having a predetermined rate-of-rise,characterized by means effective to reduce the rate-of-rise of currentin said silicon carbide blocks, said means including an elongateinductive surge-wave modifier electrically connected and rigidly securedat its inner end to said first conducting line terminal and extendingoutwardly from said one end of said housing, and a second line terminalelectrically connected to the outer end of said inductive surge-wavemodifier, said surge-wave modifier including a magnetic core comprisinga stacked group of discrete rings of magnetic material.

9. The arrester construction as in claim 8, wherein said stacked groupincludes rings of ceramic ferrite material.

10. The arrester construction as in claim 8, in which said stacked groupincludes rings of sintered nickel-zinc ferrite containing a minorproportion of the oxides of cobalt and manganese.

11. A valve-type lightning arrester construction comprising a firstelongate housin of insulatin material having an intermediate terminalmem er at one en thereof and a ground terminal member at the other endthereof, an insulator coextensive with said housing attached to saidfirst intermediate terminal member and a first line terminal membersecured at the outer end of the insulator, a second elongate housing ofinsulating material secured to said first line terminal member andextending coaxially outward therefrom, and a second line terminal membersecured to the outer end of said second elongate housing of insulatingmaterial, a voltage-responsive, surge-discharge assembly disposed withinsaid first elongate housing constituting a preferential electricaldischarge path between said intermediate terminal member and said groundterminal member, said assembly including at least one block of boundsilicon carbide, inductive surge-wave modifying means disposed withinsaid second elongate housing and electrically connecting said lineterminal members, and spark-gap means effective to sparkover upon theappearance of a predetermined voltage between (1) either or both of saidline terminal members and (2) said intermediate terminal member.

12. The arrester construction as in claim 11, wherein the surge-wavemodifying means includes a magnetic core consisting of a stacked groupof discrete rings of magnetic material.

13. The arrester construction as in claim 11, in which the inductivesurge-wave modifying means includes a magnetic core comprising a stackedgroup of discrete rings of ceramic ferrite material.

14. The arrester construction as in claim 11, wherein the inductivesurge-wave modifying means includes a core of nickelzinc ferritecontaining a minor proportion of oxides of cobalt and manganese, thecobalt oxide not exceeding 3 percent weight and the manganese oxide notexceeding 5 percent weight, based on total percent weight.

1. A valve-type lightning arrester construction comprising an elongate housing of insulating material containing a surgedischarge structure which includes a quench-spark-gap portion and at least one block of nonlinear resistance material disposed in series circuit relation between a line terminal at one end of said housing and a ground terminal at the other end of said housing, and surge-modifying means mounted externally on the line terminal end of said housing and coextensive therewith, said surge-modifying means including a casing of insulating material having an elongate cavity therein, a rectilinear throughconductor extending longitudinally through said casing, terminal means at the respective ends of said casing, a nickel-zinc ferrite core disposed within the cavity and surrounding the through-conductor, and means connecting said surge-modifying means in series circuit relation with said surge-discharge structure, said surge-modifying means being effective to inductively alter the current-time relationship of electrical surges discharging from the line terminal to the ground terminal before contacting any part of said discharge structure.
 2. A method for reducing fulgurite channeling through the nonlinear resistance blocks in the discharge path of a valve-type lightning arrester while the arrester is connected to an electrical conductor which transmits to the arrester an electrical composite of normal dynamic energy and a surge wave of abnormal superimposed energy, comprising the step of retardingly modifying the current-rate-of-rise of the surge wave at its point of entrance to the discharge path by passing the surge wave through a single-turn, magnetically saturable ferrite-cored surge-wave modifier.
 3. A valve-type lightning arrester construction comprising a pair of elongate, transversely spaced housings of insulating material containing elements constituting a voltage-responsive arc-extinguishing discharge structure including current-limiting silicon-carbide resistors each of which is susceptible to thermal damage by the sudden passage of a current surge therethrough within a predetermined time interval such that time for current deconcentration and heat dispersion from the initial current-carrying channel is unavailable, a first line terminal at one end of one of said housings connected to the discharge structure therein, a second line terminal at the adjacent one end of the other of said housings connected to the discharge structure therein, common ground-terminal means at the respective other ends of said housings connected to the respective discharge structures therein, and an inductive surge-wave modifier mechanically joining and electrically connecting said first line terminal to said second line terminal.
 4. The arrester construction defined in claim 3, in which the inductive surge-wave modifier includes a magnetic core comprising a stacked group of discrete rings of magnetic material.
 5. The arrester construction as in claim 3, wherein the inductive surge-wave modifier includes a magnetic core comprising a stacked group of discrete rings of ceramic ferrite material.
 6. The arrester construction as in claim 3, in which the inductive surge-wave modifier includes a core comprising a stacked group of discrete rings of sintered nickel-zinc ferrite containing a relatively small proportion of the oxides of cobalt and manganese.
 7. A valve-type surge arrester comprising a pair of adjacent elongate insulator housings each containing a similar series circuit assembly of current-limiting resistors and spark gaps, spaced line terminals at one pair of adjacent ends of said housings and spaced ground terminals at the other pair of adjacent ends of said housings, conducting means electrically connecting said spaced ground terminals, and conducting means electrically connecting said spaced line terminals, said last-named conducting means including a saturable inductive surge-wave modifier effective to implement surge sparkover of one of said series circuit assemblies and concurrently facilitate initiation of sparkover of the other of said series circuit assemblies.
 8. A valve-type lightning arrester construction comprising an elongate housing of insulating material, a first conducting line terminal disposed at one end of said housing and a conducting ground terminal disposed at the other end of said housing, and elements constituting a preferential electrical discharge path interposed between said line and said ground terminals, said elements including a plurality of bound silicon carbide blocks subject to thermal damage upon the passage therethrough of a current having a predetermined rate-of-rise, characterized by means effective to reduce the rate-of-rise of current in said silicon carbide blocks, said means including an elongate inductive surge-wave modifier electrically connected and rigidly secured at its inner end to said first conducting line terminal and extending outwardly from said one end of said housing, and a second line terminal electrically connected to the outer end of said inductive surge-wave modifier, said surge-wave modifier including a magnetic core comprising a stacked group of discrete rings of magnetic material.
 9. The arrester construction as in claim 8, wherein said stacked group includes rings of ceramic ferrite material.
 10. The arrester construction as in claim 8, in which said stacked group includes rings of sintered nickel-zinc ferrite containing a minor proportion of the oxides of cobalt and manganese.
 11. A valve-type lightning arrester construction comprising a first elongate housing of insulating material having an intermediate terminal member at one end thereof and a ground terminal member at the other end thereof, an insulator coextensive with said housing attached to said first intermediate terminal member and a first line terminal member secured at the outer end of the insulator, a second elongate housing of insulating material secured to said first line terminal member and extending coaxially outward therefrom, and a second line terminal member secured to the outer end of said second elongate housing of insulating material, a voltage-responsive, surge-discharge assembly disposed within said first elongate housing constituting a preferential electrical discharge path between said intermediate terminal member and said ground terminal member, said assembly including at least one block of bound silicon carbide, inductive surge-wave modifying means disposed within said second elongate housing and electrically connecting said line terminal members, and spark-gap means effective to sparkover upon the appearance of a predetermined voltage between (1) either or both of said line terminal members and (2) said intermediate terminal member.
 12. The arrester construction as in claim 11, wherein the surge-wave modifying means includes a magnetic core consisting of a stacked group of discrete rings of magnetic material.
 13. The arrester construction as in claim 11, in which the inductive surge-wave modifying means includes a magnetic core comprising a stacked group of discrete rings of ceramic ferrite material.
 14. The arrester construction as in claim 11, wherein the inductive surge-wave modifying means includes a core of nickel-zinc ferrite containing a minor proportion of the oxides of cobalt and manganese, the cobalt oxide not exceeding 3 percent weight and the manganese oxide not exceeding 5 percent weight, based on total percent weight. 